Circadian variations in aortic stiffness, sympathetic vasoconstriction, and post-ischemic vasodilation in adults with and without type 2 diabetes.


The current literature reveals a lack of information on the circadian variations of some important cardiovascular risk factors related to the work of the heart or the capacity to provide blood and oxygen to various tissues. These factors include aortic stiffness, peripheral vasoconstrictor responsiveness, and post-ischemic vasodilation capacity. Furthermore, it is not clear whether the impact of an external stressor capable of activating the sympathetic nervous system could have greater repercussions on the cardiovascular system in the morning than in the evening. Given the higher incidence of acute cardiovascular events in the morning than in the evening, the studies undertaken in this thesis aim to investigate the circadian variations of these factors that are linked to cardiovascular risk, both at rest and during acute activation of the sympathetic nervous system. Type 2 diabetes (T2DM) is a condition that induces deleterious changes in cardiovascular function, impacting cardiovascular mortality and morbidity. Thus, the impact of diabetes will be evaluated. As a secondary purpose, considering the sex differences in the incidence and prognosis of cardiovascular disease, the effect of sex will be evaluated. Aortic stiffness proved not to be increased in the morning compared to the evening at specific times when the cardiovascular risk is significantly different, both at rest and during sympathetic activation. However, while healthy older women show similar aortic stiffness values compared to their male counterparts during acute stress, older women with T2DM reported greater aortic stiffness compared to men with T2DM. The post-ischemic forearm vasodilation is blunted in the morning compared to the evening in healthy elderly and such an attenuated vasodilation capacity impairs blood flow supply towards the ischemic area. The presence of T2DM does not affect vasodilation capacity and reactive hyperemia, but induces circadian variations in arterial pressure. The peripheral vasoconstriction triggered by a standardized sympathetic stressor is similar between morning and evening, regardless of the presence of T2DM and reduced baseline vascular conductance values in the morning. However, the peripheral vasoconstriction responsiveness is blunted in individuals with T2DM than in healthy ones as sympathetic activation induces vasodilation on the contralateral forearm in individuals with T2DM and vasoconstriction in healthy age-matched subjects. This finding highlights a neurovascular response to an external stressor altered by T2DM. Taken together, our findings suggest that the baseline state of constriction of the peripheral vascular tissue is greater in the morning than in the evening, but this fact is not due to greater sympathetic vasoconstriction responsiveness in the morning. Higher morning vasoconstriction at baseline however affects the capacity of a vascular tissue to dilate and, in turn, to supply blood to an ischemic tissue. Similar sympathetic vasoconstriction responsiveness between morning and evening is a likely factor explaining similar or lower values of central artery stiffness in the morning than in the evening, not only at rest but also during sympathetic excitation. Paradoxically, adults with T2DM report an increase in sympathetic-mediated dilatation capacity on the vascular tissue, which might be a defense mechanism that allows to reduce the central pressor response during sympathetic excitation

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